Lithium ion secondary batteries have a characteristic of small size and large capacity, and are widely used as power supplies of portable telephones and notebook-sized personal computers. However, while urgent advancement of portable electronic equipments and utilization to electronic vehicles are realized in recent years, further improvements in energy density are required.
Examples of the method for improving energy density include using an active material that has high capacity, increasing an operating potential of the battery, and improving charge/discharge efficiency as well as battery cycle life. From among these, methods, method that increases an operating potential of the battery is effective for downsizing and weight-saving a battery module used for electronic vehicles or the like because an assembled battery having a smaller number of serially-connected batteries than a conventional assembled battery can be provided.
As a conventional positive electrode active material for a lithium ion secondary battery, materials such as lithium cobaltate and lithium manganite, whose operating potential is in the 4 V class (average operating potential=3.6 to 3.8 V: with respect to lithium potential) are used. This is because the developed potential is defined by oxidation and reduction reaction of Co ion or Mn ion (Co3+←→Co4+ or Mn3+←→Mn4+). In contrast, it is known, for example, that operating potential of 5 V class (average operating potential=4.6 V or more: with respect to lithium potential) can be realized by using a spinel compound obtained by substituting Mn of a lithium manganate to Ni, Co, Fe, Cu, Cr or the like as the active material. In this compound, Mn exists in a state of quaternary, and the operating potential is defined by oxidation and reduction reaction of the substituting atom instead of oxidation and reduction reaction of Mn.
The capacity of LiNi0.5Mn1.5O4 is 130 mAh/g or more, and the average operating voltage is 4.6 V or higher with respect to metal lithium, and the material is expected as a material having high energy density. Further, the spinel type lithium manganese oxide is advantageous in that it has a three-dimensional lithium spreading path, that it has thermodynamic stability higher than the other compound, and that it can be easily synthesized.
Patent Documents 1 to 2 disclose a secondary battery in which a fluorinated compound such as a fluorinated ether, a fluorinated carbonate, a fluorinated ester, a fluorinated acrylate, or a fluorinated cyclic-type carbonate is used as a solvent in the case of using a positive electrode active material showing charging and discharging field of 4.5 V or higher.
Patent Documents 3 and 4 show adding a cyclic-type sulfonate to an electrolyte liquid in order to improve the preserving property at high temperature.
Patent Document 5 discloses using an electrolyte liquid containing a cyclic-type sulfonate derivative in a battery in which a 5 V class positive electrode active material is used.
Non-Patent Document 1 shows that capacity of self-discharge during stored state is reduced by adding 1,3-propane sultone in a cell in which a 5 V class positive electrode active material and Li as the opposite electrode are used.
As described above, the effect of the fluorinated ether as a solvent that is resistant to oxidation is known, and the cyclic-type sulfonate is known as an additive for forming a stable film on an electrode. Patent Document 6 discloses an example of using an electrolyte liquid containing both a fluorinated ether and a cyclic-type sulfonate. However, although Patent Document 6 only shows the improvement of flame retardancy by the fluorinated ether in the 4 V class positive electrode and the improvement of discharge capacity ratio of high rate and low rate, it does not disclose the effect of improving operating life in the case of using a positive electrode having high potential.